The objectives of the proposed research include: 1. To elucidate some of the basic mechanisms of transport of molecules across biological membranes with a particular emphasis on the differentiation of transepithelial transport systems. Larval and adult amphibians represent a biological preparation ideally suited for such studies. 2. Two organ systems are under study--the skin and the kidney. The skin is particularly interesting because the Na plus and Cl minus transport systems do not become functional until late in metamorphosis, thus it is possible to study the morphological, biochemical, and biophysical correlates of the appearance of these systems.